In Positron Emission Tomography (PET), the emitted photons are attenuated during their passage through the object to be imaged, especially the human body. Thus artifacts occur in reconstructed PET images as well as limitations in the quantitative evaluation. To rectify this problem it is known that an attenuation correction based on an attenuation map can be carried out. In the final analysis the attenuation map specifies how large the local attenuation coefficient is at specific positions within the body. For the attenuation correction it is necessary for the exact position and density of the body parts and anatomical structures of an object under examination to be determined. For stand-alone PET systems the attenuation map can be determined for example from CT images or from external sources.
In recent times however hybrid magnetic resonance PET devices (MR-PET devices) have been developed which ideally allow parallel imaging based on MR and PET. In these devices techniques have been proposed for also using MR imaging to determine the position of anatomical features or even the tissue distribution. Furthermore it has been proposed that an attenuation map determined previously with another device also be used for MR.
In such cases however it is precisely with hybrid MR-PET devices that difficulties arise. On the one hand an MR-PET device mostly provides only a very restricted patient image, meaning an image with a small radius in which, in particular as regards the extremities, mainly the arms, patients position themselves differently. An exact, reliable and diagnostically-evaluatable MR image is in addition only possible in a restricted area in which the field exhibits a sufficient homogeneity and the gradient fields a sufficient linearity. This area frequently lies in the area of the torso. Images of the arms or if necessary also of the legs, which are located at the edge of the patient image, with diagnostic quality are thus not possible. In particular the non-linearity of the gradient fields results in distortions, but image artifacts can also arise from the lack of homogeneity. Thus there has not been any method known to date of reliably recording images in a combined MR-PET device of the position of parts of the body of a patient, and also it is only possible with difficulty—especially in the area of the arms—to obtain information needed to adapt or to determine an attenuation map.